Approximately 60 million gallons of lubricants and cutting fluids are used each year in metal cutting and metal forming operations in the United States at a cost of more than $350 million. This figure represents only a fraction of the cost associated with machines and their tools, bits, production materials, and labor. The selection of coolants or lubricants, however, is important for reasons other than cost. Particular cutting fluids are also selected for their performance: for the quality of the part produced, its accuracy and dimensional stability and finish, surface cleanliness, reduction in tool wear, corrosion protection, ease of machining, especially high-speed machining, and finally, for shop safety and environmental protection.
Materials that are processed into useful parts by cutting and forming operations include metals, alloys, plastics, ceramics, and composites. The removal processes include: turning, milling, broaching, drilling, tapping, cutoff, grinding, polishing, and lapping. These processes apply a tool or an abrasive at sufficient speed or force to remove a given quantity of material. Chip material-forming processes include: forging, rolling, extrusion, rod and wire drawing, tube drawing, deep drawing, swaging, and roll forming. These processes rely on plastic deformation of the material. In material removal processes, the rate of production as well as the life of the tool can be influenced by whether or not an effective cutting fluid is used. Ineffective cooling can lead to thermal distortion of the workpiece that subsequently produces a loss in dimensional tolerances.
The two main functions of cutting fluids are lubrication at relatively low cutting speeds and cooling at relatively high cutting speeds of the tool, chip, and workpiece. By serving these functions, cutting fluids (a) prevent tool, workpiece, and machine overheating, (b) increase tool life, (c) improve surface finish on the workpiece, and (d) help clear the swarf from the cutting area. Cutting fluids are usually either water or oil-based; the oil may be either natural or synthetic. Various methods are used to apply lubricants: dripping, flooding, high pressure jet, misting, and manual brushing. Older methods generally flood the interface area from the top down, but for efficient high-speed machining, a jet directed under the end clearance face and about the chip by high pressure spraying has been found to be very effective.
Water-based fluids have higher heat capacities than those with an oil-base and can sustain increased heat loads during high-speed machining. Water-based fluids, however, promote corrosion in some materials where oil coolants do not. Oil-based fluids normally have one-fourth to one-half the thermal capacity of water, often require higher flow rates, sometimes support bacteria growth, and may become toxic upon evaporation at high temperatures. Additives to some coolants also limit their use with certain materials; for example, lubricating fluids for iron and steel are normally not used with aluminum. Similarly, cutting fluids with chlorine-bearing additives can not be used for machining titanium alloys. There is often concern for the cleanliness of the manufactured part because of the cutting fluids.
Some materials are also more difficult to machine than others. One particularly important quality is thermal conductance--the ability to conduct the heat generated during operation away from the chip formation zones. The heat generated depends on the material and the rate of machining. Thirty to forty years ago, machining speeds for steel were 40 to 70 feet/minute; common high speed machining today approaches speeds of 1,000 feet/minute. The effectiveness of cutting or cooling fluids is related to their contact or dwell time at the tool/workpiece interface. Since this contact time is reduced at high cutting speeds, the role of the cutting fluid is much more important in high-speed machining of materials with limited thermal conductance. High-strength steels and many composites are difficult to machine at high speeds, so improved lubricating or cooling fluids aid the fabrication with these materials. Metals such as high-strength steels and high temperature alloys dull even the best tungsten carbide tools in a short time. In some cases, ceramic tools can replace tungsten carbide tools because they have higher compressive strength and can withstand higher temperatures. However, they are brittle, have low tensile strength, and cannot be used with lubricants. With some plastics and composite materials, the heat generated during high-speed machining is sufficient to plasticize the chip and workpiece.
Operator safety is also a concern in an environment where machinists can become covered with lubricating fluids. One significant threat is the generation of potentially hazardous vapors during high-speed machining. As machine speeds increase, so do the chip-tool interface temperatures that cause evaporation of the cutting fluid. The cost of replacing evaporated fluids increases with the speed of cutting; however, the costs of environmental pollution, of observing OSHA regulations, air-handling equipment, associated energy loss, fluids disposal, and exposure of the operators to increased risk can be significantly higher. Thus, the development of superior cutting fluids that would facilitate enhanced cooling of the cutting tool and workpiece, would improve lubricity and workplace safety, and reduce environmental pollution, would be of great value to the manufacturing industries.
It is, accordingly, an object of the present invention to provide a cooling fluid and method that minimizes the risk of tool, workpiece and machine overheating.
It is another object of the present invention to provide a cooling fluid and method that extends useful tool life.
Yet another object of the present invention is to provide a cooling fluid and method that improves workpiece surface finish.
Still another object of the present invention is to provide a cooling fluid and method that enhances operator safety by reducing toxic fumes.
A still further object of the present invention is to provide a cooling fluid and method that reduces environmental hazards.